Brandon is 4 times as old as Nadia and is also 21 years older than Nadia. How old is Nadia?
Explanation: We can use the given information to write down two equations that describe the ages of Brandon and Nadia. Let Brandon's current age be $b$ and Nadia's current age be $n$ $b = 4n$ $b = n + 21$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $n$ , and both of our equations have $b$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $4n$ $-$ $ (n + 21)$ which combines the information about $n$ from both of our original equations. Solving for $n$ , we get: $3 n = 21$ $n = 7$.